1. Field of the Invention
The present invention relates to a process for producing silicon carbide fibers. More particularly, the present invention relates to a process for producing silicon carbide fibers having excellent mechanical strength and heat resistance.
2. Description of the Related Art
As a simple process for producing a silicon carbide fiber, Japanese Unexamined Patent Publication No. 6-192,917 discloses a process for preparing a silicon carbide fiber by reacting a porous carbon fiber with a silicon monoxide (SiD) gas at a temperature of 800 to 2000.degree. C.
Also, Japanese Unexamined Patent Publication No. 7-97,281 discloses a process for producing a silicon carbide material having a sheet form structure or three dimensional structure by reacting porous carbon fibers having a specific surface area of 100 to 3000 m.sup.2 /g and formed into a sheet form structure or a three dimensional structure, for example, a honeycomb structure, with a silicon monoxide gas at a temperature of 800 to 2000.degree. C.
Further, Japanese Unexamined Patent Publication No. 7-277,719 discloses a process in which porous carbon fibers having a specific surface area of 100 to 2500 m.sup.2 /g and formed into a sheet form structure or a three dimensional structure are reacted with a silicon monoxide gas at a temperature of 800 to 2000.degree. C., and then the resultant silicon carbide article in the form of fibers, a sheet or a three dimensional structure is heat-treated in a gas atmosphere comprising nitrogen and substantially no oxygen.
Still further, Japanese Unexamined Patent Publication No. 10-53924 discloses a process for producing a silicon carbide fiber in which process, an activated carbon fiber having a specific surface area of 100 to 2500 m.sup.2 /g is reacted with a silicon monoxide gas at a temperature of 800 to 2000.degree. C. under a reduced pressure, and the resultant silicon carbide fiber is coated with a coating containing as a component, a metal oxide. In this process, the metal oxide includes silicon oxide and boron oxide.
This Japanese publication further discloses a process for producing a silicon carbide fiber by reacting an activated carbon fiber with a silicon monoxide gas at a temperature of 800 to 2000.degree. C. under a reduced pressure, heat treating the resultant silicon carbide fiber in an atmosphere containing, as a component, nitrogen at a temperature of 800 to 2000.degree. C., and coating the heat treated silicon carbide fiber with a coating containing, as a component, a metal oxide. In this process, the nitrogen-containing atmosphere is defined to an atmosphere containing, as a component, an nitrogen element-containing gas, for example, a nitrogen gas or ammonia gas, and substantially not containing oxygen.
The above-mentioned prior arts are disadvantageous in that the resultant silicon carbide fiber is unsatisfactory in mechanical strength for composite materials in which the silicon carbide fiber is used as a reinforcing fiber.
Accordingly, there has been a strong demand for a process for producing a silicon carbide fiber having not only a high heat resistance but also a significantly enhanced mechanical strength.